1. Field of the Invention
This invention relates generally to semiconductor devices and data reading methods therefor, and fabricating methods of such semiconductor devices.
2. Description of the Related Art
Some conventional non-volatile memories have Contents Addressable Memory (CAM) cells that store device information such as device IDs used for various tests or device identification, or trimming data for adjusting the circuit characteristics with ease (such as disclosed in Japanese Patent Application Publication No. 2004-213872).
A CAM cell is composed of a floating gate and a control gate stacked above a semiconductor substrate to form a gate with a source and a drain respectively formed in the semiconductor substrate at both sides of the gate. That is, the CAM cell has the same configuration as that of a cell transistor that serves as a memory transistor.
In recent years, miniaturization and high integration of semiconductor devices has led to more defects, resulting in reduced yield. To address this problem, a redundant circuit is sometimes provided in the semiconductor device. There has been proposed a technique in which the information on the redundant addresses, operation specifications, and other device specific information can be stored by cutting off a fuse provided in a fuse circuit in the semiconductor device (such as disclosed in Japanese Patent Application Publication No. 2004-246958).
A logic device, however, does not typically include the aforedescribed CAM, and it is problematic that the device ID or the trimming data cannot be retained. In a case where a non-volatile memory is employed for retaining the information, there are additional problems in that more steps are necessary during wafer processing for such fuse programming circuit and the scale of the programming circuit is large relative to the memory capacity.
In a semiconductor device having the abovedescribed fuse, if the fuse, which is made of a metal, is not cut off completely, the difference in voltage applied to both ends of the fuse gradually decreases the resistance value of the fuse. This is called grow back effect. When the resistance value of the fuse is lowered to less than a given value, the fuse circuit judges that the fuse is not melted down. This causes a problem in that the fuse cannot be melted down successfully. With respect to a laser fuse, an underlying layer is necessary in consideration of the damage applied by the laser to the semiconductor substrate and, thus, a cover film needs to be opened. This is also problematic in view of reliability.